Treatment system, calibration method, and storage medium

ABSTRACT

A treatment system of embodiments includes an imaging system including one or more radiation sources and a plurality of detectors, a first acquirer, a second acquirer, a first deriver, a second deriver, and a calibrator. The radiation sources radiate radiation to an object in a plurality of different directions. The plurality of detectors detect the radiation at different positions. The first acquirer acquires images based on the radiation. The second acquirer acquires position information of a first imaging device in a three-dimensional space. The first deriver derives the position of the object in the images. The second deriver derives the position of a second imaging device in the three-dimensional space based on the position of the object in the images, the position of the first imaging device, and the like. The calibrator performs calibration of the imaging system based on a derivation result of the second deriver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2018-140736, filed Jul. 26, 2018 andInternational Application No. PCT/JP2019/029176, filed Jul. 25, 2019;the entire contents all of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a treatment system, acalibration method, and a storage medium.

BACKGROUND

In treatment using radiation, it is necessary to perform calibration ofan imaging system such as a flat panel detector (FPD) and an X-ray tubebefore treatment in order to realize high-accuracy positioning and tumortracking for a patient. With respect to this, a technology in which aphantom with a marker having a known position in a three-dimensionalspace of a treatment room is installed at a predetermined position inthe treatment room, the installed phantom is imaged with radiation, theposition of the marker is derived from the captured image, and animaging system is calibrated based on the marker position derived fromthe image and the known marker position in the three-dimensional spaceis known.

However, in the conventional technology, there are cases in whichcalibration with high accuracy cannot be performed when thethree-dimensional position of the marker is not known or the knownthree-dimensional position of the marker includes an error.Consequently, there are cases in which the position of a test objectsuch as a patient cannot be determined with high accuracy or a targetsuch as a tumor cannot be tracked with high accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of a treatment system in a firstembodiment.

FIG. 2 is an external view of a treatment apparatus in the firstembodiment.

FIG. 3 is a diagram showing an example of a configuration of a medicalimage processing apparatus in the first embodiment.

FIG. 4 is a diagram showing an example of a configuration of acalibration processor.

FIG. 5 is a flowchart showing an example of calibration processing.

FIG. 6 is a diagram schematically showing a state in which radiation isradiated from two radiation sources.

FIG. 7 is a diagram showing a method of deriving a three-dimensionalposition of a radiation source using a bundle adjustment method.

FIG. 8 is a flowchart showing another example of the calibrationprocessing.

FIG. 9 is a diagram showing an example of a screen displayed on adisplay.

FIG. 10 is a diagram showing an example of treatment plan data.

FIG. 11 is a diagram showing a transparent image selection method basedon a treatment plan.

FIG. 12 is a diagram showing a transparent image selection method basedon a treatment plan.

DETAILED DESCRIPTION

According to one embodiment, a treatment system includes an imagingsystem including one or more radiation sources and a plurality ofdetectors as imaging devices, a first acquirer, a second acquirer, afirst deriver, a second deriver, and a calibrator. The radiation sourcesradiate radiation to a certain object in a plurality of differentdirections. The plurality of detectors detect the radiation radiatedfrom the radiation sources at different positions. The first acquireracquires a plurality of images based on the radiation detected by theplurality of detectors. The second acquirer acquires positioninformation representing at least one of the position and the directionof a first imaging device included in the imaging system in athree-dimensional space in which the imaging system is disposed. Thefirst deriver derives the position of the object in each of theplurality of images acquired by the first acquirer. The second deriverderives at least one of the position and the direction of a secondimaging device included in the imaging system in the three-dimensionalspace based on the position of the object in the images derived by thefirst deriver and the position or the direction of the first imagingdevice represented by the position information acquired by the secondacquirer. The calibrator performs calibration of the imaging systembased on a derivation result of the second deriver.

According to the present embodiment, it is possible to provide atreatment system, a calibration method, and a storage medium capable ofdetermining the position of a test object with high accuracy andtracking a target with high accuracy.

Hereinafter, a treatment system, a calibration method, and a storagemedium of embodiments will be described with reference to the drawings.

First Embodiment

FIG. 1 is a diagram showing an example of a treatment system 1 in afirst embodiment. For example, the treatment system 1 includes atreatment apparatus 10, a terminal apparatus 20, and a medical imageprocessing apparatus 100. These apparatuses are connected through anetwork NW. The network NW includes, for example, the Internet, a widearea network (WAN), a local area network (LAN), a provider terminal, awireless communication network, a wireless base station, a dedicatedline, and the like. All combinations of the apparatuses shown in FIG. 1need not communicate with each other and a part of the network NW mayinclude a local network.

The treatment apparatus 10 is an apparatus which radiates firstradiation as a beam (hereinafter referred to as a treatment beam B) to atest object OB in any direction of 360° with the test object OB as acenter. The test object OB is a patient who receives treatment using thetreatment beam B, for example. The first radiation includes, forexample, particle radiation such as a heavy particle beam, an electronbeam, a proton beam, and a neutron beam, and electromagnetic radiationsuch as X-rays and y-rays. The treatment apparatus 10 radiates secondradiation to generate a tomographic image of the test object OB in orderto check the position of the test object OB. The second radiationincludes, for example, electromagnetic radiation such as X-rays.Hereinafter, an example in which the first radiation (treatment beam B)is a “heavy particle beam” and the second radiation is “X-rays” will bedescribed.

The terminal apparatus 20 is used by a user (hereinafter referred to asa maintenance operator U) who performs maintenance such as preparationand maintenance, repairing, inspection and trimming of the treatmentapparatus 10. For example, the terminal apparatus 20 may be a terminalapparatus including an input device, a display device, a communicationdevice, a storage device, and an arithmetic device, such as a cellularphone such as a smartphone, a tablet terminal, and various personalcomputers. The communication device of the terminal apparatus 20includes a network card such as a network interface card (NIC), awireless communication module, and the like.

The medical image processing apparatus 100 tracks a target movingaccording to the respiration and movement of the heartbeat of a patientthat is a test object OB and radiates the treatment beam B to thetreatment apparatus 10 with respect to the tracked target at anappropriate timing. The target is an organ such as a lung or the liver.Tracking of this target is performed based on a tomographic image of thetest object OB captured using X-rays or the like in a stage prior to atreatment stage and a transparent image obtained by imaging the testobject OB in the treatment stage.

The medical image processing apparatus 100 may derive a position gapbetween the position of the test object OB in the treatment stage andthe position of the test object OB when a treatment plan has been formedand provide information about the derived position gap to a person (adoctor or the like) who performs radiation treatment using the treatmentsystem 1.

FIG. 2 is an external view of the treatment apparatus 10 in the firstembodiment. The treatment apparatus 10 in the first embodiment includes,for example, a bed 11, an arm 11 a, a plurality of radiation sources(radiation radiating holes) 12, two detectors 13-1 and 13-2, a radiatinggate 14, a sensor 15, and a treatment apparatus controller 16. The twodetectors 13-1 and 13-2 are an example of “detectors.”

The plurality of radiation sources 12, the two detectors 13-1 and 13-2,and the radiating gate 14 are provided in a housing in a ring shape(torus shape) which is called a rotary gantry G. For example, when thevertical direction of a three-dimensional space representing a room(hereinafter referred to as a treatment room) in which the treatmentapparatus 10 is installed is denoted by Z_(f), one side of thehorizontal direction is denoted by X_(f) and the other side is denotedby Y_(f), the rotary gantry G is installed with the direction Y_(f) as arotation axis such that it can rotate 360° on the rotation axis. Therotary gantry G is an example of an “imaging system” and each of theplurality of radiation sources 12 and the two detectors 13-1 and 13-2provided in the rotary gantry G is an example of an “imaging device.”The radiating gate 14 is an example of a “particle beam source.”

The test object OB is fixed to the bed 11. The arm 11 a has one endfixed to the floor or the like of the treatment room and the other endfixed to the bed 11 and moves the bed 11 from the outside to the insideor from the inside to the outside of the rotary gantry G in a state inwhich the bed 11 is separated from the floor or the like of thetreatment room.

The plurality of radiation sources 12 are arranged at predeterminedintervals in a circumferential direction of the rotary gantry G, forexample. Each radiation source 12 radiates, for example, X-rays to theinner circumferential side of the rotary gantry G. Accordingly, when thebed 11 moves to the inside of the rotary gantry G, for example, X-raysare radiated to the test object OB fixed to the bed 11 in a plurality ofdifferent directions of 360°. A radiation generation device (not shown)that generates X-rays may be installed outside the treatment room.

The detectors 13-1 and 13-2 detect, for example, X-rays radiated fromthe radiation sources 12. For example, the detectors 13-1 and 13-2include a rectangular detector such as a flat panel detector (FPD), animage intensifier or a color image intensifier. For example, thedetectors 13-1 and 13-2 convert an analog signal based on detectedX-rays into a digital signal and output the digital signal to themedical image processing apparatus 100 as a transparent image TI. Thetransparent image TI is a two-dimensional image and is a tomographicimage of the test object OB. The number of detectors provided in therotary gantry G is not limited to 2 and may be 3 or more.

The radiating gate 14 is provided at a certain position in thecircumferential direction of the rotary gantry G. The radiating gate 14radiates the treatment beam B to the inner circumferential side of therotary gantry G. Although a single radiating gate 14 is provided in therotary gantry G in the example of FIG. 1 , the present invention is notlimited thereto. For example, a plurality of radiating gates may beprovided in the rotary gantry G. A radiation generation device (notshown) that generates the treatment beam B may be installed outside thetreatment room.

The sensor 15 is a sensor which detects movement of an affected partaccording to respiration of a patient as a phase when the test object OBis the patient. For example, the sensor 15 is a pressure sensor. In thiscase, the sensor 15 may be attached to the body of the patient. Thesensor 15 outputs information representing the detected phase of therespiration of the patient as a waveform to the medical image processingapparatus 100.

The treatment apparatus controller 16 is realized, for example, by ahardware processor such as a central processing unit (CPU) or a graphicsprocessing unit (GPU) executing a program (software) stored in a storagedevice (not shown) such as a read only memory (ROM). The treatmentapparatus controller 16 may be realized by hardware (circuit: circuitry)such as a large scale integration (LSI) circuit, an application specificintegrated circuit (ASIC), or a field-programmable gate array (FPGA), orrealized by software and hardware in cooperation.

The treatment apparatus controller 16 is controlled by the medical imageprocessing apparatus 100 to operate the plurality of radiation sources12, the detectors 13-1 and 13-2, and the radiating gate 14. Thetreatment apparatus controller 16 is controlled by the medical imageprocessing apparatus 100 to rotate the rotary gantry G.

FIG. 3 is a diagram showing an example of a configuration of the medicalimage processing apparatus 100 in the first embodiment. The medicalimage processing apparatus 100 in the first embodiment includes, forexample, a communicator 102, an inputter 104, a display 106, a medicalimage processing apparatus controller 110, and a storage 130.

The communicator 102 includes, for example, a communication interfacesuch as an NIC. The communicator 102 communicates with the treatmentapparatus 10 and the terminal apparatus 20 through the network NW andreceives various types of information. The communicator 102 outputs thereceived information to the medical image processing apparatuscontroller 110. The communicator 102 may be controlled by the medicalimage processing apparatus controller 110 to transmit information to thetreatment apparatus 10 and the terminal apparatus 20 connected throughthe network NW. The communicator 102 is an example of an “outputter.”

For example, the inputter 104 receives an input operation from a usersuch as a doctor or a nurse and outputs a signal based on the receivedinput operation to the medical image processing apparatus controller110. For example, the inputter 104 is realized by a mouse, a keyboard, atrackball, a switch, a button, a joystick, a touch panel, or the like.The inputter 104 may be realized, for example, by a user interface thatreceives audio input, such as a microphone. When the inputter 104 is atouch panel, the display 106 which will be described later may beintegrated with the inputter 104.

The display 106 displays various types of information. For example, thedisplay 106 displays an image generated by the medical image processingapparatus controller 110 or displays a graphical user interface (GUI) orthe like for receiving an input operation from an operator. For example,the display 106 is a liquid crystal display (LCD), an organicelectroluminescence (EL) display, or the like. The display 106 isanother example of the “outputter.”

The medical image processing apparatus controller 110 includes, forexample, a first acquirer 112, a second acquirer 114, an image processor116, a treatment beam radiation controller 118, an information outputcontroller 120, and a calibration processor 122. The treatment apparatuscontroller 16 and the treatment beam radiation controller 118 are anexample of a “radiation controller.”

These components are realized by a hardware processor such as a CPU or aGPU executing a program (software) stored in the storage 130. Some orall of these components may be realized by hardware (circuit: circuitry)such as an LSI, an ASIC or an FPGA or software and hardware incooperation. The aforementioned program may be stored in the storage 130in advance or stored in a detachable storage medium such as a DVD or aCD-ROM and installed into the storage 130 from the storage medium whenthe storage medium is inserted into a drive device of the medical imageprocessing apparatus 100.

The storage 130 is realized, for example, by a storage device such as aROM, a flash memory, a random access memory (RAM), a hard disc drive(HDD), a solid state drive (SSD) or a register. The flash memory, HDD,SSD, and the like are non-transient storage media. These non-transientstorage media may be realized by other storage devices connected throughthe network NW, such as a network attached storage (NAS) and an externalstorage server device. For example, four-dimensional tomographic imagedata 132, treatment plan data 134, and the like are stored in thestorage 130. These will be described later.

For example, the four-dimensional tomographic image data 132 istime-series arrangement of n three-dimensional tomographic images (CTimages) that are three-dimensional volume data. The three-dimensionaltomographic images are captured in a treatment planning stage, forexample. A period obtained by multiplying n by the time interval of thetime-series images is set such that it covers a period in which arespiration phase changes by one cycle, for example. The respirationphase is a phase having a period from when a patient exhales and theninhales to when the patient exhales again as one cycle. For example,n=10. For example, an area indicating the outline of a tumor that is anaffected part, an area indicating the outline of an organ to which thetreatment beam B is not desired to be radiated, and the like are set byan input operation of a doctor or the like in the image area of at leastone of the n three-dimensional tomographic images. In otherthree-dimensional tomographic images, the same areas as the areas of theoutlines set by the input operation of the doctor or the like areautomatically set according to deformable registration. The deformableregistration is processing of expanding position information (theoutline of the organ, or the like in the above case) designated withrespect to three-dimensional volume data at a certain point in time tothree-dimensional volume data at a different point in time fortime-series three-dimensional volume data.

The treatment plan data 134 is data representing a treatment plan formed(planned) in the treatment planning stage. The treatment plan is, forexample, a plan in which a treatment beam B radiation direction such asa direction in which the treatment beam B will be radiated when apatient that is a test object OB is positioned at a position, and theintensity of the treatment beam B when the treatment beam B is radiated,and the like have been determined for each patient that is a test objectOB. This treatment plan may be planned based on a treatment method suchas a gated radiation method or a tracking radiation method.

The first acquirer 112 acquires, for example, transparent images TI fromthe detectors 13-1 and 13-2 through the communicator 102. Since thetransparent images TI are generated by the detectors 13-1 and 13-2 inreal time during treatment, for example, the first acquirer 112 acquirestransparent images TI continuing in time series.

The second acquirer 114 acquires position information representingpositions or directions (positions or directions in thethree-dimensional space of the treatment room) of one or more of theplurality of imaging devices provided in the rotary gantry G. Forexample, the three-dimensional positions or directions of the imagingdevices are measured by a laser tracker. Here, it is assumed that theposition of the laser tracker is determined as a relative position basedon an object (e.g., the rotation axis of the rotary gantry G, or thelike) that is the origin in the three-dimensional space of the treatmentroom.

In the present embodiment, as an example, imaging devices havingpositions or directions measured by the laser tracker will be describedas the detectors 13-1 and 13-2. The imaging devices having positions ordirections measured by the laser tracker may be the radiation sources12. For example, when the positions and directions of the detectors 13-1and 13-2 are measured, a medical professional such as a doctor or anurse obtains relative positions of a plurality of probes by installingthe probes (e.g., reflectors and the like) that easily reflect laserbeams at three or more points of the detection planes of the detectors13-1 and 13-2 and measuring the probes using the laser tracker. Then,the medical professional derives the positions and directions of thedetectors 13-1 and 13-2 based on the relative positions of the pluralityof probes. An imaging device having the position or direction measuredby the laser tracker is an example of a “first imaging device.”

When the three-dimensional positions of the imaging devices have beenmeasured by the laser tracker, the second acquirer 114 acquires positioninformation representing the three-dimensional positions of the imagingdevices from the laser tracker through the communicator 102. When adoctor or the like inputs position information to the inputter 104, thesecond acquirer 114 may acquire the information input to the inputter104 as position information representing the three-dimensional positionsof imaging devices. Measurement of the positions or directions ofimaging devices is not limited to measurement using the laser trackerand the positions or directions of imaging devices may be measured usinga stereo camera or a contact type sensor, for example.

The image processor 116 determines the position of the test object OB.For example, the image processor 116 generates a digitally reconstructedradiograph (DRR) based on a three-dimensional tomographic image of eachrespiration phase included in four-dimensional tomographic image data132 of each test object OB stored in the storage 130. A DRR is a virtualtransparent image generated from three-dimensional volume data inresponse to a virtual radiation source when it is assumed that radiationis radiated from the virtual radiation source to the three-dimensionaltomographic image (three-dimensional volume data).

For example, the image processor 116 generates a DRR when thethree-dimensional tomographic image has been viewed at a viewpoint inthe same direction as a radiation direction of X-rays radiated to thecurrent test object OB using a method called 3D-2D registration based onthe three-dimensional tomographic image of each respiration phaseincluded in the four-dimensional tomographic image data 132, thetransparent image TI on the side of the detector 13-1 and thetransparent image TI on the side of the detector 13-2 acquired by thefirst acquirer 112. When the image processor 116 generates the DRR, theimage processor 116 may generate the DRR that is a virtualtwo-dimensional tomographic image by rendering a three-dimensionaltomographic image using a ray casting method. Here, the image processor116 may integrate each element value of the three-dimensionaltomographic image and use the integrated value as an element value ofeach element of the DRR or use a maximum value of each element value ofthe three-dimensional tomographic image as an element value of eachelement of the DRR.

For example, the image processor 116 selects a DRR corresponding to athree-dimensional tomographic image of an exhalation phase from DRRscorresponding to three-dimensional tomographic images of respirationphases as a template image. The exhalation phase is a tomographic imagecaptured in a state in which the patient that is the test object OB hasexhaled.

The image processor 116 compares the DRR selected as the template imagewith transparent images TI sequentially acquired by the first acquirer112 and performs matching of the position of a target (an organ or thelike). When the positions of the target in the DRR and the transparentimages TI match, the image processor 116 determines that the respirationphase (exhalation phase) of the three-dimensional image that is thesource of the DRR corresponds to the current respiration phase of thepatient and permits radiation of the treatment beam B. Here, the imageprocessor 116 may further permit radiation of the treatment beam B whenthe position of the target is within a radiation area determined inadvance. The radiation area may be arbitrarily determined by a medicalprofessional, for example.

The treatment beam radiation controller 118 causes the radiating gate 14to radiate the treatment beam B to the test object OB at a positiondetermined by the image processor 116 when the image processor 116permits radiation of the treatment beam B. For example, the treatmentbeam radiation controller 118 extracts information such as a radiationangle of the treatment beam B and an intensity of the treatment beam Bfrom the treatment plan indicated by the treatment plan data 134 andoutputs various types of extracted information to the treatmentapparatus controller 16. Upon reception of this, the treatment apparatuscontroller 16 causes the rotary gantry G to rotate or causes theradiating gate 14 to radiate the treatment beam B.

The information output controller 120 causes the display 106 to displayan image or causes the communicator 102 to transmit information, forexample, in response to presence or absence of permission to radiate thetreatment beam B.

The calibration processor 122 performs calibration of the rotary gantryG. FIG. 4 is a diagram showing an example of a configuration of thecalibration processor 122. The calibration processor 122 includes, forexample, a first deriver 122 a, a second deriver 122 b, and a calibrator122 c.

Hereinafter, processing of each component of the calibration processor122 will be described using a flowchart. FIG. 5 is a flowchart showingan example of calibration processing. Processing of this flowchart maybe repeatedly performed in a first cycle. The first cycle is, forexample, a period of approximately one month or several months.

First, a medical professional or the like derives the positions anddirections of the detectors 13-1 and 13-2 using a laser tracker or thelike (step S100).

Next, the second acquirer 114 acquires position information representingthe positions and directions of the detectors 13-1 and 13-2 in thethree-dimensional space (step S102). Accordingly, the positions anddirections of the detectors 13-1 and 13-2 in the three-dimensional spaceare handled as known information during calibration.

Next, the medical professional or the like sets a phantom having four ormore markers embedded therein inside the rotary gantry G such that thephantom is projected in a transparent image T1 (step S104). The phantomis an acrylic case in a cubic shape, for example.

A marker may be any object that attenuates X-rays, such as an iron ballor a wire, for example. At least one of the four or more markers isembedded in the phantom such that it is present on a plane differentfrom a plane (two-dimensional space) on which the other three or moremarkers are present in the three-dimensional space in the phantom.Accordingly, a space formed when the markers embedded in the phantom areused as vertices is a three-dimensional space. It is assumed that thepositions of these markers and positional relationships between themarkers are known in advance.

The medical professional or the like may dispose an object associatedwith the treatment apparatus 10 such as the bed 11 and the arm 11 ainside the rotary gantry G instead of setting the phantom.

Next, the medical professional or the like inputs information aboutcompletion of setting of the phantom to the inputter 104. Upon receptionof this information, the treatment apparatus controller 16 of thetreatment apparatus 10 selects two radiation sources 12 from theplurality of radiation sources 12 and causes the two selected radiationsources 12 to radiate X-rays in a plurality of different directions(step S106).

FIG. 6 is a diagram schematically showing a state in which the tworadiation sources 12 are caused to radiate radiation. In the figure,12-1 represents one radiation source of the two selected radiationsources 12 and 12-2 represents the other radiation source of the twoselected radiation sources 12. A broken line r-1 represents X-raysradiated from the radiation source 12-1 and a broken line r-2 representsX-rays radiated from the radiation source 12-2. PH represents a phantomand MK indicates a marker. 14 a represents a radiation hole (heavyparticle source) of the treatment beam B radiated from the radiatinggate 14.

For example, the treatment apparatus controller 16 causes the rotarygantry G to rotate such that an angle around the rotation axis of therotary gantry G becomes a certain angle θ1 and causes the radiationsources 12-1 and 12-2 to radiate X-rays. Next, the treatment apparatuscontroller 16 causes the rotary gantry G to rotate such that the anglearound the rotation axis of the rotary gantry G becomes an angle θ2shifted from the angle θ1 by a predetermined angle (e.g., 15°) andcauses the radiation sources 12-1 and 12-2 to radiate X-rays. When therotary gantry G is rotated, the position of the phantom PH (markers MK)imaged using X-rays does not change. In this manner, the treatmentapparatus controller 16 causes the detectors 13-1 and 13-2 to generatetransparent images TI capturing the phantom PH in a plurality ofdirections by repeatedly causing the radiation sources 12-1 and 12-2 toradiate X-rays while causing the rotary gantry G to rotate for eachpredetermined angle width (step S108). For example, when the radiationsources 12-1 and 12-2 radiate X-rays while the angle is changed by 15°in the 360° omni-direction, each of the detectors 13-1 and 13-2generates 24 transparent images TI (a total of 48 corresponding to thesum of images of the two detectors).

Next, the first acquirer 112 acquires a plurality of transparent imagesTI from the detectors 13-1 and 13-2 through the communicator 102 (stepS110).

Next, the first deriver 122 a derives positions of the markers MK withrespect to each of the plurality of transparent images TI acquired bythe first acquirer 112 (step S112).

For example, the first deriver 122 a derives the positions of themarkers MK by template-matching a template image prepared in advance andthe transparent images T1. An image of the markers MK captured inadvance, an image generated through simulation, or the like may be usedas the template image. When the shape of the markers MK is known, thefirst deriver 122 a may scan a shape filter for extracting the shape ofthe markers MK through raster scan or the like for the transparentimages T1 and derive positions having high degrees of matching with theshape filter as the positions of the markers MK.

Next, the second deriver 122 b derives three-dimensional positions ofthe radiation sources 12-1 and 12-2 which are unknown parameters basedon the positions of the markers MK derived from the transparent imagesTI and the position information acquired by the second acquirer 114(step S114). When imaging devices having positions or directionsmeasured by the laser tracker are radiation sources 12, the secondderiver 122 b may derive the positions and directions of the detectors13-1 and 13-2 which are unknown parameters. An imaging device having athree-dimensional position derived by the second deriver 122 b is anexample of a “second imaging device.”

For example, when the markers MK are imaged in multiple directions whilethe rotary gantry G is rotated, three-dimensional positions of themarkers MK that are imaging targets are unknown but the positionsthereof are invariant. Accordingly, the three-dimensional position ofeach object can be represented as a common parameter between transparentimages TI. For example, the second deriver 122 b may derive thepositions of the radiation sources 12-1 and 12-2 by applying a method ofderiving three-dimensional positions and parameters of an imaging systembased on feature points corresponding to each other between multi-viewimages, such as bundle adjustment. The method called bundle adjustmentis a method of adjusting all unknown parameters such that, when markersMK are re-projected onto an image using estimated imaging systemparameters, positions of the re-projected markers MK correspond topositions of the markers detected from the image as much as possible.

FIG. 7 is a diagram showing a method of deriving a three-dimensionalposition of a radiation source 12 according to the bundle adjustmentmethod. For example, when the number of markers MK embedded in thephantom PH is N, a three-dimensional position of each marker MK isrepresented as X^(i)(→)=(X^(i), Y^(i), Z^(i))^(t) as in the illustratedexample. “i” is any natural number from 1 to N, t representstransposition, and (→) represents a vector. The position of each markerMK derived from M sets of transparent images TI acquired by imaging at Mtypes of angles is represented as x^(ij)(→)=(x^(ij), x^(ij))^(t). “j” isany natural number from 1 to M.

When a projection matrix by which the three-dimensional positionX^(i)(→) of each marker MK is projected to the plane of a j-thtransparent image TI is set to P^(j)(→), the position at which an i-thmarker MK should be projected onto the j-th transparent image TI isrepresented as x^(ij)(˜)(→)=(x^(ij)(˜), y^(ij)(˜))^(t). (˜) representsthe tilde symbol. For example, the second deriver 122 b derives theposition (two-dimensional position in the image) x^(ij)(˜)(→) of thei-th marker MK projected onto the j-th transparent image TI by solvingmathematical expression (1).

$\begin{matrix}\left\lbrack {{Expression}1} \right\rbrack & \end{matrix}$ $\begin{matrix}{{\lambda\begin{bmatrix}{\overset{\sim}{x}}^{ij} \\1\end{bmatrix}} = {P^{j}\begin{bmatrix}X^{i} \\1\end{bmatrix}}} & (1)\end{matrix}$

For example, the second deriver 122 b searches for various types ofparameters that minimize the total (hereinafter referred to as are-projection error) of sums of squares of differences (variances)between positions x^(ij)(˜)(→) to which the respective markers MK shouldbe projected and derived positions x^(ij)(→) of the respective markersMK through bundle adjustment.

For example, the second deriver 122 b derives the projection matrixP^(j)(→) based on a base vector R^(j)(→)=(u^(j), v^(j), w^(j)) of adetector 13 when the j-th transparent image TI has been captured, acenter position C^(j)(→)=(c_(x) ^(i), c_(y) ^(i), c_(z) ^(i))^(t) of adetection plane of the detector 13 (a center position of the transparentimage TI), and a three-dimensional position T^(j)(→)=(l_(x) ^(i), l_(y)^(i), l_(z) ^(i))^(t) of a radiation source 12. The center positionC^(j)(→) of the detection plane of the detector 13 is athree-dimensional position of ((w−1)/2, (h−1)/2) when the width of thetransparent image T1 is w and the height is h.

For example, the second deriver 122 b derives the projection matrixP^(j)(→) using the above-described parameters based on mathematicalexpression (2).

$\begin{matrix}\left\lbrack {{Expression}2} \right\rbrack & \end{matrix}$ $\begin{matrix}{P^{j} = {\begin{bmatrix}\frac{f}{s_{u}} & 0 & x_{c} \\0 & \frac{f}{s_{v}} & y_{c} \\0 & 0 & 1\end{bmatrix}\left\lceil {R^{j^{t}} - {R^{j^{t}}T^{j}}} \right\rceil}} & (2)\end{matrix}$

In mathematical expression (2), f represents a distance from thethree-dimensional position of the radiation source 12 to thethree-dimensional position of the detector 13, s_(u) and s_(v) representpixel pitches of axes (u, v) of the transparent image TI, and x_(c) andy_(c) represent an intersection position in the image when an opticalaxis w intersects the detection plane of the detector 13. The distance fcan be represented as mathematical expression (3) and the intersectionposition (x_(c), y_(c)) can be represented as mathematical expression(4).

$\begin{matrix}\left\lbrack {{Expression}3} \right\rbrack & \end{matrix}$ $\begin{matrix}{f = {w\left\lbrack {C - T} \right\rbrack}} & (3)\end{matrix}$ $\begin{matrix}\left\lbrack {{Expression}4} \right\rbrack & \end{matrix}$ $\begin{matrix}{\begin{bmatrix}x_{c} \\y_{c}\end{bmatrix} = {\begin{bmatrix}\frac{w - 1}{2} \\\frac{h - 1}{2}\end{bmatrix} + {\begin{bmatrix}\frac{1}{s_{u}} & 0 \\0 & \frac{1}{s_{v}}\end{bmatrix}\begin{bmatrix}{u\left\lbrack {C - T} \right\rbrack} \\{v\left\lbrack {C - T} \right\rbrack}\end{bmatrix}}}} & (4)\end{matrix}$

The second deriver 122 b derives the re-projection error based onmathematical expression (5). Since the positions and directions of thedetectors 13-1 and 13-2 in the three-dimensional space are known, asdescribed above, parameters to be optimized are the three-dimensionalposition T^(j)(→) of the radiation source 12 and the three-dimensionalpositions X^(i)(→) of the markers MK.

$\begin{matrix}\left\lbrack {{Expression}5} \right\rbrack & \end{matrix}$ $\begin{matrix}{\left( {{\hat{T}}^{1},{\cdots{\hat{T}}^{M}},{\hat{X}}^{1},{\cdots{\hat{X}}^{N}}} \right) = {\underset{T^{1},{\cdots T}^{M},X^{1},{\cdots X}^{N}}{argmin}\frac{1}{MN}{\sum\limits_{i = 1}^{M}{\sum\limits_{j = 1}^{N}{\left\lbrack {x^{ij} - {\overset{\sim}{x}}^{ij}} \right\rbrack^{t}\left\lbrack {x^{ij} - {\overset{\sim}{x}}^{ij}} \right\rbrack}}}}} & (5)\end{matrix}$

For example, the second deriver 122 b may perform optimization of thetwo types of parameters of the three-dimensional position T^(j)(→) ofthe radiation source 12 and the three-dimensional positions X^(i)(→) ofthe markers MK using an optimization method called particle swarmoptimization. Particle swarm optimization is an optimization method thatimitates behaviors of a large group of insects and a method of searchingfor optimum positions while updating positions and speeds of particlesduring communication between particles by causing particles to haveposition and speed information of a search space. Although the positionand direction of the detector 13 are assumed to be known in the presentembodiment, when the three-dimensional position of the radiation source12 is assumed to be known, the second deriver 122 b may employ thecenter position C^(j)(→) of the detection plane of the detector 13 (thecenter position of the transparent image TI), the base vector R^(j)(→),and three-dimensional positions X^(i)(→) of the markers MK as theparameters to be optimized. In this manner, the position of each imagingdevice provided in the rotary gantry G can be estimated even when knownparameters are any of the positions of the detectors 13 or the radiationsources 12. The known parameters may be any of the positions anddirection of the detectors 13 and the positions of the radiation sources12 or parameters corresponding to combinations thereof.

Next, the second deriver 122 b causes the storage 130 to store thederived three-dimensional positions T^(j)(→) of the radiation sources 12(12-1 and 12-2) as reference positions of the radiation sources 12 whichwill be referred to in the next and following processes (step S116).Accordingly, processing of this flowchart ends. The reference positionsof the radiation sources 12 are, for example, parameters referred toduring calibration performed in a second cycle shorter than the firstcycle. The second cycle is, for example, a period of approximately oneday or several days.

Hereinafter, calibration performed in the second cycle will be describedusing a flowchart. FIG. 8 is a flowchart showing another example of thecalibration processing. Processing of this flowchart is repeatedlyperformed in the second cycle, for example.

First, a medical professional or the like sets a phantom having four ormore markers embedded therein inside the rotary gantry G such that thephantom is projected to a transparent image TI (step S200).

Next, the medical professional or the like inputs information aboutcompletion of setting of the phantom to the inputter 104. Upon receptionof this information, the treatment apparatus controller 16 of thetreatment apparatus 10 selects two radiation sources 12 from theplurality of radiation sources 12 and causes the two selected radiationsources 12 to radiate X-rays in a plurality of different directions(step S202).

Next, the detectors 13-1 and 13-2 generate transparent images TI of thephantom PH in a plurality of directions (step S204).

Next, the first acquirer 112 acquires a plurality of transparent imagesTI from the detectors 13-1 and 13-2 through the communicator 102 (stepS206).

Next, the first deriver 122 a derives positions of markers MK withrespect to each of the plurality of transparent images TI acquired bythe first acquirer 112 (step S208).

Next, the second deriver 122 b derives three-dimensional positions ofthe radiation sources 12-1 and 12-2 which are unknown parameters basedon the positions of the markers MK derived from the transparent imagesTI and the position information acquired by the second acquirer 114(step S210).

Next, the calibrator 122 c derives differences between thethree-dimensional positions of the radiation sources 12-1 and 12-2derived by the second deriver 122 b and the reference positions of theradiation sources 12-1 and 12-2 stored in the storage 130 and determineswhether the differences are equal to or greater than a threshold value(step S212).

When known parameters are the three-dimensional positions of theradiation sources 12-1 and 12-2, the three-dimensional positions of thedetectors 13-1 and 13-2 derived in calibration in the first cycle arestored as reference positions and three-dimensional directions thereofare stored as reference directions in the storage 130. In this case, thesecond deriver 122 b derives the three-dimensional positions and thethree-dimensional directions of the detectors 13-1 and 13-2 which areunknown parameters as processing of S210. Accordingly, when the knownparameters are the three-dimensional positions of the radiation sources12-1 and 12-2, the calibrator 122 c may derive differences between thethree-dimensional positions of the detectors 13-1 and 13-2 derived bythe second deriver 122 b and the reference positions and differencesbetween the three-dimensional directions of the detectors 13-1 and 13-2derived by the second deriver 122 b and the reference directions anddetermine whether each difference is equal to or greater than athreshold value.

The information output controller 120 outputs an alarm representing thatmaintenance is necessary to the medical professional or the like usingthe treatment apparatus 10 when the calibrator 122 c determines that thedifferences are equal to or greater than the threshold value (stepS214).

For example, the information output controller 120 causes the display106 to display an image as shown in FIG. 9 as an exemplary example of analarm. FIG. 9 is a diagram showing an example of a screen displayed onthe display 106. As in the illustrated example, text or an imagerepresenting that maintenance is required may be displayed on the screenof the display 106.

The information output controller 120 may output a mail or pushnotification for requesting maintenance to the terminal apparatus 20 asan alarm through the communicator 102.

Meanwhile, the calibrator 122 c performs calibration based ondifferences from the reference positions (or reference directions) upondetermining that the differences are less than the threshold value (stepS216).

For example, the calibrator 122 c may use the differences from thereference positions (or reference directions) as a correction amount andperform geometric transformation, such as an affine transformation, of aDRR as calibration based on the correction amount. The calibrator 122 cmay perform an affine transformation of transparent images T1 ascalibration instead of or in addition to using the differences from thereference positions (or reference directions) as a correction amount andperforming an affine transformation of a DRR based on the correctionamount.

The calibrator 122 c may use the differences from the referencepositions (or reference directions) as a correction amount and correctparameters referred to when a DRR is generated based on the correctionamount as calibration. The parameters include, for example, the positionand direction of each imaging device provided in the rotary gantry G.More specifically, the aforementioned parameters include the positionsand/or directions of the detectors 13-1 and 13-2 and the positions ofthe radiation sources 12.

When the treatment apparatus 10 includes an adjustment mechanism whichautomatically controls the positions of the radiation sources 12 and thedetectors 13, the calibrator 122 c may adjust the position and directionof each imaging device by controlling the adjustment mechanism.According to such calibration, a DDR with high accuracy can begenerated.

Although the treatment apparatus 10 is a treatment apparatus employingthe rotary gantry G in the above description, the present invention isnot limited thereto. For example, the treatment apparatus 10 may be atreatment apparatus in which the positions of imaging devices such asthe radiation sources 12 are fixed (fixed port type).

When there are statistics in which a re-projection error is small in acertain direction and large in a certain direction, the calibrationprocessor 122 may perform calibration based on transparent images TIcaptured in a direction in which the re-projection error decreases.

According to the above-described first embodiment, it is possible tocalibrate the imaging system of the treatment apparatus 10 with highaccuracy even when three-dimensional positions of markers MK are notknown or known three-dimensional positions of markers MK include anerror by including the rotary gantry G including one or more radiationsources 12 which radiate radiation to a certain object in a plurality ofdifferent directions and a plurality of detectors 13 which detect theradiation radiated from the radiation sources 12 at different positionsas imaging devices, the first acquirer 112 which acquires a plurality oftransparent images TI based on radiation detected by the plurality ofdetectors 13, the second acquirer 114 which acquires positioninformation of the radiation sources 12 or the detectors 13, the firstderiver 122 a which derives positions of markers MK in a phantom PH ineach of the plurality of transparent images TI acquired by the firstacquirer 112, the second deriver 122 b which derives a three-dimensionalposition and the like of an imaging device for which positioninformation has not been acquired based on the positions of the markersMK in the transparent images TI derived by the first deriver 122 a andthree-dimensional positions of the radiation sources 12 orthree-dimensional positions and three-dimensional directions of thedetectors 13 represented by the position information acquired by thesecond acquirer 114, and the calibrator 122 c which performs calibrationof the rotary gantry G based on a derivation result of the secondderiver 122 b. As a result, it is possible to determine the position ofa test object with high accuracy and track a target with high accuracyduring treatment.

In general, radiation with sufficient power needs to be correctlyradiated to an affected part of a patient when radiation treatment isperformed. Accordingly, the patient is positioned by comparing an imageof the patient acquired when treatment is planned with an image of thepatient captured when radiation is radiated, and with respect to anaffected part moving due to respiration, the affected part is trackedthrough transparent images TI according to X-rays after positioning andradiation is radiated thereto. To position this patient and performaffected part tracking with high accuracy, the imaging system needs tohave been calibrated. However, when three-dimensional positions ofmarkers MK are unknown or known three-dimensional positions of markersMK include an error, there are cases in which the position of a testobject cannot be determined with high accuracy or a target cannot betracked with high accuracy.

In particular, when calibration is performed, a larger error thanassumed is easily generated in known three-dimensional positions ofmarkers MK because the phantom PH is set in a frame and imaged. Forexample, in the case of a treatment apparatus in which the rotary gantryG is not employed and the positions of imaging devices such as theradiation sources 12 and the like are fixed, it is conceivable that aframe in a rectangular parallelepiped shape is installed on the floor ofa treatment room and the phantom PH is disposed on the frame. However,in the case of the treatment apparatus 10 employing the rotary gantry Gas in the present embodiment, there are cases in which it is difficultto install a frame in a rectangular parallelepiped shape on the floorbecause the radiation sources 12 is laid on the floor of the treatmentroom. Accordingly, there are cases in which an L-shaped frame isinstalled at a position separated from positions at which the radiationsources 12 are laid, for example. Here, the L-shaped frame is installedon the floor in such a manner that one side of the L-shaped edges isgrounded to the floor and the other side is hanging in the air. Forexample, the phantom PH is disposed on the edge on the side hanging inthe air between the edges of the L-shaped frame. In the L-shaped frame,minute distortion such as downward warping of the edge on the sidehanging in the air in the vertical direction may be generated due tomoment of force. In such a case, an error in the position of the phantomPH easily becomes larger than an originally assumed error. On the otherhand, even when the phantom PH is disposed on the bed 11 or the likewithout preparing a frame, a dimensional error and the like of the arm11 a connected from the floor to the bed are integrated and thus anerror in the position of the phantom PH easily becomes larger than theoriginally assumed error. In this manner, an error is easily generatedin the position of the phantom PH when the phantom PH is set in thetreatment room. When an error is generated in the position of thephantom PH, the three-dimensional position and three-dimensionaldirections of the detectors 13 and the three-dimensional positions ofthe radiation sources 12 also include errors and thus calibrationaccuracy easily decreases.

In contrast, in the present embodiment, the three-dimensional positionof any imaging device provided in the rotary gantry G is measured inadvance using a laser tracker and thus an unknown three-dimensionalposition of an imaging device can be derived even when positions ofmarkers MK are not known. As a result, it is possible to calibrate theimaging system of the treatment apparatus 10 with high accuracy,determine the position of a test object with high accuracy and track atarget with high accuracy.

The above-described medical image processing apparatus 100 may berealized by a general apparatus including a processor such as a CPU or aGPU and a storage device such as a ROM, a RAM, an HDD or a flash memory,the storage device storing a program for causing the processor to serveas the rotary gantry G including one or more radiation sources 12 whichradiate radiation to a certain object in a plurality of differentdirections and a plurality of detectors 13 which detect the radiationradiated from the radiation sources 12 at different positions as imagingdevices, the first acquirer 112 which acquires a plurality oftransparent images TI based on radiation detected by the plurality ofdetectors 13, the second acquirer 114 which acquires positioninformation of the radiation sources 12 or the detectors 13, the firstderiver 122 a which derives positions of markers MK in a phantom PH ineach of the plurality of transparent images TI acquired by the firstacquirer 112, the second deriver 122 b which derives a three-dimensionalposition and the like of an imaging device for which positioninformation has not been acquired based on the positions of the markersMK in the transparent images T1 derived by the first deriver 122 a andthree-dimensional positions of the radiation sources 12 orthree-dimensional positions and three-dimensional directions of thedetectors 13 represented by the position information acquired by thesecond acquirer 114, and the calibrator 122 c which performs calibrationof the rotary gantry G based on a derivation result of the secondderiver 122 b.

Second Embodiment

Hereinafter, a second embodiment will be described. The secondembodiment differs from the above-described first embodiment in thattransparent images T1 that are position derivation targets duringcalibration are selected based on a treatment plan of a patient.Hereinafter, a description will focus on differences from the firstembodiment and description of common points in the first and secondembodiments will be omitted. In a description of the second embodiment,the same parts as those in the first embodiment are denoted by the samereference signs and described.

FIG. 10 is a diagram showing an example of the treatment plan data 134.For example, the treatment plan data 134 is information in which atreatment date and time is associated with a treatment plan such as aradiation angle θ of the treatment beam B radiated during treatment foreach patient.

The first deriver 122 a in the second embodiment selects a transparentimage TI that is a target from which positions of markers MK will bederived from a plurality of transparent images TI acquired by the firstacquirer 112 and derives the positions of the markers MK in the selectedtransparent image TI.

For example, when a timing at which calibration in the second cycle isperformed is set to early morning of each day, the first deriver 122 aselects a patient scheduled to receive treatment after calibration froma plurality of patients scheduled to receive treatment in a treatmentplan. For example, when the treatment plan is the one illustrated inFIG. 10 and a timing at which calibration in the second cycle isperformed is “early morning Jun. 1, 2020,” the first deriver 122 aselects patients A, B and C scheduled to receive treatment on that day.Then, the first deriver 122 a selects transparent images TI that aretargets from which positions of markers MK will be derived based onradiation angles θ of treatment beams B associated with the selectedpatients A, B and C.

FIG. 11 and FIG. 12 are diagrams showing a method of selecting atransparent image TI based on a treatment plan. For example, when anangle between a horizontal direction X_(f) that passes through therotation axis center and a radiation direction of the treatment beam Bof the radiating gate 14 is assumed to be a radiation angle θ of thetreatment beam B in the angle around the rotation axis of the rotarygantry G, the first deriver 122 a selects transparent images TTgenerated based on X-rays radiated from the radiation sources 12-1 and12-2 when the rotary gantry G has been rotated at the angle θ. Forexample, in the case of a patient A, the first deriver 122 a selects atransparent image TI capturing markers MK according to X-rays radiatedfrom the radiation sources 12-1 and 12-2 when the phantom PH includingthe markers MK is disposed inside the rotary gantry G and the rotarygantry G has been rotated at an angle at which the treatment beam B isradiated to the patient A from right above, as shown in FIG. 12 ,because the radiation angle θ of the treatment beam B is 90°.Accordingly, it is possible to omit calibration with respect toradiation directions of the treatment beam B which are not used fortreatment on that day.

According to the above-described second embodiment, it is possible toreduce a time required for calibration compared to a case in which 360°omni-directional calibration is performed because transparent images T1that are position derivation targets during calibration are selectedbased on a treatment plan of a patient.

According to at least one of the above-described embodiments, it ispossible to calibrate the imaging system of the treatment apparatus 10with high accuracy even when three-dimensional positions of markers MKare not known or known three-dimensional positions of markers MK includean error by including the rotary gantry G which includes, as imagingdevices, one or more radiation sources 12 which radiate radiation to acertain object in a plurality of different directions and a plurality ofdetectors 13 which detect the radiation radiated from the radiationsources 12 at different positions, the first acquirer 112 which acquiresa plurality of transparent images TI based on radiation detected by theplurality of detectors 13, the second acquirer 114 which acquiresposition information of the radiation sources 12 or the detectors 13,the first deriver 122 a which derives positions of markers MK in aphantom PH in each of the plurality of transparent images TI acquired bythe first acquirer 112, the second deriver 122 b which derives athree-dimensional position and the like of an imaging device for whichposition information has not been acquired based on the positions of themarkers MK on the transparent images TI derived by the first deriver 122a and three-dimensional positions of the radiation sources 12 orthree-dimensional positions and three-dimensional directions of thedetectors 13 represented by the position information acquired by thesecond acquirer 114, and the calibrator 122 c which performs calibrationof the rotary gantry G based on a derivation result of the secondderiver 122 b. As a result, it is possible to determine the position ofa test object with high accuracy and track a target with high accuracyduring treatment.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A treatment system comprising: an imaging systemcomprising, as imaging devices, one or more radiation sources which areconfigured to radiate radiation to a certain object in a plurality ofdifferent directions, and a plurality of detectors which are configuredto detect the radiation radiated from the radiation sources at differentpositions; a first acquirer which is configured to acquire a pluralityof images based on the radiation detected by the plurality of detectors;a second acquirer which is configured to acquire position informationrepresenting at least one of a position and a direction of a firstimaging device included in the imaging system in a three-dimensionalspace in which the imaging system is disposed; a first deriver which isconfigured to derive a position of the object in each of the pluralityof images acquired by the first acquirer; a second deriver which isconfigured to derive at least one of a position and a direction of asecond imaging device included in the imaging system in thethree-dimensional space based on the position of the object in theimages derived by the first deriver and the position or the direction ofthe first imaging device represented by the position informationacquired by the second acquirer; and a calibrator which is configured toperform calibration of the imaging system based on a derivation resultof the second deriver.
 2. The treatment system according to claim 1,wherein the imaging system is installed in a rotary gantry, and thetreatment system further comprises a radiation controller which isconfigured to cause the rotary gantry to rotate and cause the radiationsources to radiate radiation while changing positions of the radiationsources with respect to the object.
 3. The treatment system according toclaim 1, wherein the object includes four or more markers, wherein atleast one of the four or more markers is present on a second planedifferent from a first plane on which the other three or more markersare present in the three-dimensional space in the object.
 4. Thetreatment system according to claim 1, wherein the second deriver isfurther configured to derive the position of the object in thethree-dimensional space based on the position of the object in theimages derived by the first deriver and the position or the direction ofthe first imaging device represented by the position informationacquired by the second acquirer.
 5. The treatment system according toclaim 1, further comprising a particle beam source which is configuredto radiate a particle beam different from radiation radiated from theradiation sources to a test object, wherein the first deriver isconfigured to select an image that is a target from which the positionof the object will be derived from the plurality of images acquired bythe first acquirer based on a treatment plan associated with a radiationdirection of the particle beam radiated by the particle beam source foreach test object, and derive the position of the object in the selectedimage.
 6. The treatment system according to claim 5, wherein the firstdriver is configured to select the test object to which the particlebeam is scheduled to be radiated after calibration of the imaging systemfrom a plurality of test objects to which the particle beam is scheduledto be radiated in the treatment plan, and wherein an image based on theradiation radiated in the same direction as a radiation direction of theparticle beam associated with the selected test object is selected fromthe plurality of images acquired by the first acquirer as an image thatis a target from which the position of the object will be derived. 7.The treatment system according to claim 1, further comprising an imageprocessor which is configured to generate a virtual two-dimensionalimage when viewed at a certain view based on three-dimensional imagesobtained by arranging images based on the radiation detected by thedetectors in the radiation direction of the radiation, wherein thecalibrator is configured to perform a geometric transformation of thetwo-dimensional image generated by the image processor as thecalibration based on the derivation result of the second deriver.
 8. Thetreatment system according to claim 1, wherein the calibrator isconfigured to perform a geometric transformation of the images acquiredby the first acquirer as the calibration based on the derivation resultof the second deriver.
 9. The treatment system according to claim 1,further comprising an image processor which is configured to generate avirtual two-dimensional image when viewed at a certain view based onthree-dimensional images obtained by arranging images based on theradiation detected by the detectors in the radiation direction of theradiation, wherein the calibrator is configured to perform correction ofparameters referred to when the image processor generates thetwo-dimensional image as the calibration based on the derivation resultof the second deriver.
 10. The treatment system according to claim 1,further comprising: an outputter which outputs information; and anoutput controller which is configured to control the outputter to outputinformation for requesting adjustment of the position or the directionof the imaging system to a terminal apparatus of a user who performsmaintenance of the imaging system when a first difference between theposition of the second imaging device in the three-dimensional spacederived by the second deriver and a reference position or a seconddifference between the direction of the second imaging device in thethree-dimensional space derived by the second deriver and a referencedirection is equal to or greater than a threshold value.
 11. Thetreatment system according to claim 10, wherein the second deriver isconfigured to repeat derivation of at least one of the position and thedirection of the second imaging device in the three-dimensional space ineach of a first cycle and a second cycle shorter than the first cycle,the reference position is a position of the second imaging device in thethree-dimensional space derived by the second deriver in the firstcycle, the reference direction is a direction of the second imagingdevice in the three-dimensional space derived by the second deriver inthe first cycle, the first difference is a difference between theposition of the second imaging device in the three-dimensional spacederived by the second deriver in the second cycle and the referenceposition, and the second difference is a difference between thedirection of the second imaging device in the three-dimensional spacederived by the second deriver in the second cycle and the referencedirection.
 12. A calibration method by a computer which is configured tocontrol an imaging system comprising, as imaging devices, one or moreradiation sources which are configured to radiate radiation to a certainobject in a plurality of different directions, and a plurality ofdetectors which are configured to detect the radiation radiated from theradiation sources at different positions, the calibration methodcomprising: acquiring a plurality of images based on the radiationdetected by the plurality of detectors; acquiring position informationrepresenting at least one of a position and a direction of a firstimaging device included in the imaging system in a three-dimensionalspace in which the imaging system is disposed; deriving a position ofthe object in each of the plurality of acquired images; deriving atleast one of a position and a direction of a second imaging deviceincluded in the imaging system in the three-dimensional space based onthe position of the object in the derived images and the position or thedirection of the first imaging device represented by the acquiredposition information; and performing calibration of the imaging systembased on the derived position or direction of the second imaging devicein the three-dimensional space.
 13. A non-transitory computer-readablestorage medium storing a program for causing a computer which controlsan imaging system comprising, as imaging devices, one or more radiationsources which are configured to radiate radiation to a certain object ina plurality of different directions, and a plurality of detectors whichare configured to detect the radiation radiated from the radiationsources at different positions to execute: processing of acquiring aplurality of images based on the radiation detected by the plurality ofdetectors; processing of acquiring position information representing atleast one of a position and a direction of a first imaging deviceincluded in the imaging system in a three-dimensional space in which theimaging system is disposed; processing of deriving a position of theobject in each of the plurality of acquired images; processing ofderiving at least one of a position and a direction of a second imagingdevice included in the imaging system in the three-dimensional spacebased on the position of the object in the derived images and theposition or the direction of the first imaging device represented by theacquired position information; and processing of performing calibrationof the imaging system based on the derived position or direction of thesecond imaging device in the three-dimensional space.